Hydraulic tensioners are widely used to suppress backlash and vibration due to a changes in tension in timing belts and timing chains. A typical hydraulic tensioner comprises a plunger slidable in, and protruding from, a housing. The plunger and housing cooperate to provide a high pressure oil chamber into which oil is introduced through a ball-type check valve. FIGS. 9 and 10 show a conventional hydraulic tensioner 500 having an air venting feature. In the tensioner, a hollow plunger 520 is slidable in, and protrudes from, a hole of a housing 510, the plunger being biased in the protruding direction by a spring. A pressure relief valve assembly 540 is incorporated within the protruding end of the hollow plunger 520. The pressure relief valve assembly includes an air vent disc 550, having a spiral groove 551 formed in one of its faces. The grooved face of the air vent disc abuts the end wall of the interior of the plunger, forming a spiral passage through which air, mixed with the oil inside the high pressure oil chamber, can be exhausted to the outside of the tensioner through an opening formed in the end of the plunger. The air venting mechanism of the tensioner of FIGS. 9 and 10 is described in U.S. Pat. 6,435,993.
In the tensioner of FIGS. 9 and 10, it is extremely difficult to set the optimum clearance established by the end wall of the interior of the plunger and the spiral groove in the air vent disc 550. Furthermore, the ball 541 of the relief valve assembly 540 can allow excessive flow of oil out of the tensioner when the plunger is pushed into the housing 510, preventing the required high pressure from being maintained in the housing. Another problem with the tensioner of FIGS. 9 and 10 is that the pressure relief valve assembly 540 and the air vent disc 550 are separately manufactured parts, and must be handled separately in the process of assembly of the tensioner. Consequently, assembly of the tensioner is difficult, and its cost of manufacture is high.
Objects of this invention are to solve the above-mentioned problems, and to provide a hydraulic tensioner in which high pressure in the housing is reliably maintained when required, in which release of air mixed with oil in the high pressure oil chamber occurs only when a negative pressure condition exists inside the oil chamber. It is also an object of the invention to provide a tensioner which has superior endurance and strength, and which can be assembled easily.